An MRI apparatus that places a test subject in a homogenous magnetic field space for conducting a medical diagnosis according to a nuclear magnetic resonance (referred to as “NMR”) signal is utilized in a medical institution, just like using a CT apparatus and an RI apparatus to conduct a conventional examination. In particular, an examination using the MRI apparatus shows a superior capability for detecting a lesion. This enhanced lesion detection capability is achieved because an MRI image enables various examination methods to produce not only a diagnostic image being simply morphologic, but also an image representing functional diagnostic information such as metabolism.
In order to acquire an MRI image that is medically effective from the MRI apparatus, there is required a static magnetic field generating means, enabling a long-term and stable maintenance of a high magnetic field and a homogeneous magnetic field space including deviation of a high PPM order. As a static magnetic field generator satisfying these necessary conditions as described above, a superconducting magnet is employed, which is configured in such a manner as accommodating a superconducting coil within a cryostat.
There are two types for the superconducting magnet; a cylindrical superconducting magnet which accommodates a superconducting solenoid coil within a cylindrical cryostat, and an open superconducting magnet in which the cryostats divided into two parts are placed in such a manner as opposed to each other, and a superconducting loop coil is accommodated concentrically within each of the cryostats (see patent document 1).
In any type of the superconducting magnet, there is a problem that heat penetration into the cryostat due to conduction and radiation, vaporizes and consumes a liquid helium refrigerant used for cooling the superconducting coil. Therefore, in order to prevent the consumption of the liquid helium, there is developed a superconducting magnet which incorporates a cryo-cooler having coolability for recondensing the helium gas which has been vaporized once (see patent document 2).
In order to implement a closed cryostat in which the cryo-cooler being incorporated completely condenses the vaporized helium gas to convert the helium gas back to the liquid helium, it is necessary to understand quantitatively, a heat penetration amount into the cryostat, a coolability of the cryo-cooler in the state of being incorporated into the cryostat, and a temporal change of the coolability.
This is because, the coolability of the cryo-cooler is deteriorated over time, and it is necessary to suppress the helium consumption, in association with the heat penetration amount into the cryostat and the deterioration of the coolability of the cryo-cooler. In addition, in order to maintain the coolability of the cryo-cooler, an overhaul cleaning operation is required at established intervals (typically, once every 10,000 hours running time). In the cleaning operation, the cryo-cooler is changed to a new one as resetting, or the cryo-cooler is again incorporated in the cryostat after the maintenance works are finished. Such resetting or incorporating operations include works for ensuring a thermal contact between a low-temperature part in the cryo-cooler and a heat shielding plate within the cryostat, and a quality of the works has a great influence on the coolability of the overall system. Therefore, in order to suppress the helium consumption, it is necessary to quantitatively grasp the coolability of the cryo-cooler in the state of being incorporated in the cryostat, not the coolability of an independent cryo-cooler.
There is a conventional method for grasping the coolability for overall system of the cryostat incorporating the cryo-cooler, in which the level variation of the liquid helium is observed by the use of a liquid-level meter being incorporated in the cryostat.
On the other hand, as a heat penetration amount measuring apparatus for the cryostat, there is suggested a system including a precision pressure regulator which keeps a pressure within a liquid storage tank to an arbitrary value, the regulator being provided at the outlet of a gas discharging tube for discharging gas vaporized from the refrigerant toward outside of the liquid storage tank, wherein, the heat penetration amount of the cryostat is obtained by measuring a flow amount of vaporized gas that is discharged through the gas discharging tube (see patent document 3, for example).
[Patent Document 1]
    Japanese Unexamined Patent Application Publication No. 2002-336216[Patent Document 2]    Japanese Unexamined Patent Application Publication No. 2005-237417[Patent Document 3]    Japanese Unexamined Patent Application Publication No. H05-172924